Interpretive Summary: Vesicular stomatitis virus (VSV) affects cattle, horses and pigs. The virus is spread by biting flies, causes a disease of significant economic impact in horses and in cattle and pigs, clinical signs can be a confounded for those of foot-and-mouth disease. The mechanisms of this disease have not been well elucidated. For example despite the fact that is causes lesiosn in mouth and feet, there is no evidence of blood invasion and little is known regarding virus distribution during early infection in livestock. Here we inoculated Holstein steers on the feet with VS New Jersey virus, either by directly scratching the surface of the skin (scarification) or by allowing infected black flies (Simulium vittatum) to feed in the same areas. Our findings confirmed that in both cases the virus remained localized to the inoculated skin and lymphatic nodes draining the inoculated legs (i.e. no virus was found in the blood) and lesions appeared only in the infected leg. Control animals inoculated on the skin of the neck did not develop any lesions. Animals infected by insects developed more severe lesions suggesting that factors contained in the insect saliva facilitate virus growth and lesion development. Taken together, the results indicate that VSV infection is dependent on the site of inoculation and that insects play an important role in transmission and severity of disease in cattle. This information is useful in guiding prevention and control methods during VSV outbreaks.

Technical Abstract:
Vesicular stomatitis viruses are the causative agents of vesicular stomatitis, an economically important contagious disease of livestock that occurs in North, Central, and South America. Little is known regarding the early stages of infection in natural hosts. Twelve adult Holstein steers were inoculated with Vesicular stomatitis New Jersey virus (VSNJV) on the coronary bands (CB) of the feet via scarification (SC) or by VSNJV-infected black fly (Simulium vittatum) bite (FB). Three additional animals were inoculated on the neck skin using FB. Clinical disease and lesion development were assessed daily, and animals were euthanatized from 12 hours post inoculation (HPI) through 120 HPI. The animals inoculated in the neck failed to develop any clinical signs or gross lesions, and VSNJV was detected neither by in situ hybridization (ISH) nor by immunohistochemistry (IHC). Lesions on the CB were more severe in the animals infected by FB than by SC. In both groups, peak VSNJV replication occurred between 24 and 48 HPI in keratinocytes of the CB, as evidenced by ISH and IHC. There was evidence of viral replication limited to the first 24 HPI in the local draining lymph nodes, as seen through ISH. Successful infection via FB required logarithmically less virus than with the SC technique, suggesting that components in black fly saliva may facilitate VSNJV transmission and infection in cattle. The lack of lesion development in the neck with the same method of inoculation used in the CB suggests that specific characteristics of the CB epithelium may facilitate VSNJV infection.